U.S. patent number 4,056,742 [Application Number 05/682,130] was granted by the patent office on 1977-11-01 for transducer having piezoelectric film arranged with alternating curvatures.
This patent grant is currently assigned to Tibbetts Industries, Inc.. Invention is credited to George C. Tibbetts.
United States Patent |
4,056,742 |
Tibbetts |
November 1, 1977 |
Transducer having piezoelectric film arranged with alternating
curvatures
Abstract
A transducer of the type converting between electrical and
mechanical energy by means of the piezoelectric effect in a
supported sheet of uniaxially oriented, electrically polarized thin
high polymer film such as polyvinylidene fluoride, having surface
electrodes thereon for connection to an electrical circuit. The
transducer is characterized by an arrangement of the piezoelectric
film into a series of elongate curved cylindrical segments of
alternating sign of curvature, with the surface electrodes on the
film being divided in selected locations between adjacent segments.
A useful level of elastic stability is achieved without using a
static pressure difference on the film, good electromechanical
coupling is attained, and the individual transducer elements formed
by the divided surface electrodes may be usefully interrelated
electrically to substantially cancel even order harmonic distortion
and enhance linearity of operation.
Inventors: |
Tibbetts; George C. (Camden,
ME) |
Assignee: |
Tibbetts Industries, Inc.
(Camden, ME)
|
Family
ID: |
24738344 |
Appl.
No.: |
05/682,130 |
Filed: |
April 30, 1976 |
Current U.S.
Class: |
310/357; 310/334;
310/366; 310/367; 310/800; 381/173 |
Current CPC
Class: |
B06B
1/0688 (20130101); H01L 41/0926 (20130101); H04R
17/005 (20130101); Y10S 310/80 (20130101) |
Current International
Class: |
B06B
1/06 (20060101); H01L 41/09 (20060101); H04R
17/00 (20060101); H01L 041/04 () |
Field of
Search: |
;310/8.2,8.3,8.5,8.6,9.1,9.4,9.8,9.6,9.5 ;179/11A |
References Cited
[Referenced By]
U.S. Patent Documents
Primary Examiner: Budd; Mark O.
Claims
I claim:
1. A transducer of the type converting between electrical and
mechanical energy by means of the piezoelectric effect in a
supported sheet of uniaxially oriented, electrically polarized high
polymer film having surface electrodes thereon for connection to an
electrical circuit, characterized in that:
the piezoelectric film is arranged with a series of curved
substantially cylindrical segments of alternating sign of
curvature,
the film being fixedly supported in locations between adjacent
segments and being freestanding between said locations, whereby
each curved segment functions as a distinct electromechanical
transducer, and
the surface electrodes on the film are divided in selected
locations between adjacent segments,
whereby the series of individually supported curved segments
imparts useful elastic stability to the piezoelectric film, and the
individual transducers formed by the supported curved segments and
divided surface electrodes may be interconnected electrically to
obtain desired transfer and impedance characteristics in the
transducer.
2. A transducer as claimed in claim 1 wherein the surface
electrodes are divided to place an adjacent pair of segments of
opposite curvature electrically in series.
3. A transducer as claimed in claim 2 further comprising a
non-adjacent pair of curved segments connected electrically in
parallel.
4. A transducer as claimed in claim 1 further comprising
interconnections placing the curved segments electrically in
parallel.
5. A transducer as claimed in claim 1 wherein the curved segments
are substantially congruent one to another in cross section, and
wherein the divided surface electrodes are electrically
interconnected to cause even order harmonic distortions associated
with individual curved segments to be placed in opposition to
reduce the total even order harmonic distortion of the
transducer.
6. A transducer as claimed in claim 5 wherein the curved segments
are even in number and the even order harmonic distortion
substantially cancels.
7. A transducer as claimed in claim 1 wherein the curved segments
are elongate and substantially are portions of circular
cylinders.
8. A transducer as claimed in claim 1 further comprising means
providing additional support to the the piezoelectric film, said
means comprising spaced surfaces conforming to the alternating
curved configuration of the film and means for securing the film to
the spaced surfaces.
9. A transducer as claimed in claim 8 wherein the piezoelectric
film is under tension between the spaced surfaces.
10. A transducer as claimed in claim 8 wherein support for the
piezoelectric film in locations between adjacent segments is
provided by ribs extending along the segments and between the
additional support means.
11. A transducer as claimed in claim 1 wherein the piezoelectric
film is supported about its periphery as well as in locations
between adjacent curved segments, and the curved segments are
elongate, whereby effectively inactive regions of the film near the
supported ends of the segments are small in area relative to the
total area of the segments.
12. An electroacoustic transducer utilizing the piezoelectric
effect in a supported sheet of uniaxially oriented, electrically
polarized high polymer film having surface electrodes thereon for
connection to an electrical circuit, characterized by:
means for supporting the film comprising a frame for supporting the
film about its periphery and having a pair of spaced surfaces which
comprise a series of curved substantially cylindrical segments
having alternating curvatures to which the film is conformingly
secured, thereby to form a corresponding series of curved
substantially cylindrical segments of alternating sign of curvature
in the film, and ribs extending between the pair of spaced surfaces
for supporting the film in locations between adjacent segments,
whereby each curved segment functions as a distinct electroacoustic
transducer element and useful elastic stability is conferred upon
the piezoelectric film, and
the surface electrodes on the film are divided in selected
locations between adjacent segments to allow electrical connections
to the transducer elements in a prescribed manner.
13. An electroacoustic transducer as claimed in claim 12 further
comprising a casing to which the support means is mounted, the
casing providing at least one acoustic cavity coupled to the
piezoelectric film.
14. An electroacoustic transducer as claimed in claim 13 further
comprising an acoustic connection through a wall of the casing and
extending to an acoustic cavity.
15. An electroacoustic transducer as claimed in claim 12 wherein
the curved segments are similar one to another, and further
comprising electrical connections to the transducer elements to
cause even-order harmonic distortions associated with individual
transducer elements to be placed in opposition to reduce the total
even-order harmonic distortion in an electroacoustic transfer
characteristic of the transducer.
16. An electroacoustic transducer as claimed in claim 15 wherein
the curved segments are even in number and the even-order harmonic
distortion substantially cancels.
17. An electroacoustic transducer as claimed in claim 12 wherein
the curved segments are elongate and substantially are portions of
circular cylinders.
18. An electroacoustic transducer as claimed in claim 12 wherein
the piezoelectric film is under tension between said pair of spaced
surfaces.
19. A transducer utilizing the piezoelectric effect in a supported
sheet of uniaxially oriented, electrically polarized high polymer
film having surface electrodes thereon for connection to an
electrical circuit, characterized in that:
the piezoelectric film is in the form of a series of segments of
substantially arcuate cross-section and alternating curvature,
a plurality of ribs extends along the segments for supporting the
film in locations between adjacent segments,
whereby each segment functions as a distinct transducer element and
useful elastic stability is conferred upon the piezoelectric film,
and
the surface electrodes on the film are divided in selected
locations between adjacent segments to allow electrical connections
to the transducer elements in a prescribed manner.
20. A transducer as claimed in claim 19 wherein the segments are
similar to one another, and further comprising electrical
connections to the transducer elements to cause even-order harmonic
distortions associated with individual transducer elements to be
placed in opposition to reduce the total even-order harmonic
distortion of the transducer.
21. A transducer as claimed in claim 20 wherein the segments are
even in number and the even-order harmonic distortion substantially
cancels.
22. A transducer as claimed in claim 19 wherein the segments are
elongate and substantially are portions of circular cylinders.
Description
BACKGROUND OF THE INVENTION
1. Field of the Invention
This invention relates to electromechanical or electroacoustical
transducers, and more specifically to such transducers which
convert between electrical and mechanical (including acoustical)
energy by means of the piezoelectric effect available in thin high
polymer films, such as polyvinylidene fluoride, which have been
uniaxially oriented and electrically polarized, and have surface
electrodes thereon for connection to electrical circuits.
2. Description of the Prior Art
Since its initial discovery through the work of Kawai, reported in
8 Japan. J. Appl. Phys. 975-976 (1969), the development of the
piezoelectric effect in thin high polymer films by means of
uniaxial orientation and subsequent electrical polarization has
resulted in electromechanical coupling coefficients exceeding 15%.
As with the work of Kawai this work has concentrated on
polyvinylidene fluoride (abbreviated PVF.sub.2), but improved
materials can be expected in the future, as well as further
improvement in the coupling coefficient of PVF.sub.2.
The application of such films to practical transducers has been
hindered by the unusual mechanical characteristics of the films
compared with conventional piezoelectric materials and the forms in
which they have been available. That is, the thinness and the low
elastic modulus of the films present new problems in transducer
structure, while at the same time these same characteristics,
combined with the low mass/area ratios of the films, offer the
potential for greatly improved transducer performance in several
areas of application.
In applying piezoelectric films to use in electromechanical or
electroacoustic transducers, the recent art has arranged the film
in a primitive shell configuration, such as a cylinder or a portion
of a cylinder, to transform between (1) strain in the film along
the uniaxial direction (which corresponds to the largest
piezoelectric effect) and tangent to the film surface, and (2) the
motion normal to the film surface necessary if direct
electroacoustic transduction and the accompanying low mass/area
ratio are to be achieved.
In a cylindrical shell, for example, an acoustic pressure
difference between the surfaces of the piezoelectric film is
supported by the arch of the cylindrical form and is transformed in
part into stress and strain tangential to the film and in the
uniaxial direction. Because of the electromechanical coupling
coefficient k of the film, a signal voltage is generated between
the electrodes on the film. Conversely, if an electrical signal is
applied to the electrodes, strain is generated by the piezoelectric
effect in the uniaxial direction, and the cylindrical form of the
film changes by deflections normal to its surface, resulting in the
output of acoustical energy.
However, the film is very thin, typically 8 to 30 microns, and its
elastic modulus is low, typical of organic polymers. Thus elastic
instability can set in at very low pressure differences, resulting
in unacceptable harmonic distortion, failure of the frequency
response to be approximately independent of signal level, and lack
of reproducibility of performance characteristics in general. For
example, an airborne shock wave or other acoustic overload can
irreversibly damage or change the form of the film even to the
point of substantially reversing its curvature. For these reasons,
the level of elastic stability attainable with this configuration
is insufficient for practical use.
One technique that has been used to supply elastic stability is
that of mechanically biasing the transducer, i.e., by providing a
static pressure on one side of the film so as to produce tension in
the film in the uniaxial direction. Typically this pressure is
supplied mechanically by a pad of flexible foam, held under
compression by a perforated backing plate to cause it to exert
pressure on the underside of the curved film, which consequently is
placed under static tension. The acoustically active vibration of
the film adds a dynamic component to the tension, but elastic
stability is more than assured if the total tension does not
reverse sign to become compressive.
The outstanding disadvantage of using compressed foam to develop a
mechanical bias to procure elastic stability is the deleterious
effect of the incremental stiffness of the foam on the effective
electroacoustic coupling coefficient of the transducer. In most
practical transducers there will be an air volume coupled to one
side of the film and the acoustical compliance of this space is one
of the most basic parameters that restricts the performance of the
transducer. The incremental stiffness of the foam markedly
decreases the effective acoustical compliance of this space,
without any corresponding reduction in the space occupied by the
transducer. Furthermore, the foam adds effective mass to that of
the film, and most of the film's low mass/area ratio advantage is
lost.
Another prior art solution providing mechanical bias by means of
the tractive force of an electrical field is disclosed in U.S. Pat.
No. 3,894,198, but this device requires a combination of a
piezoelectric thin film transducer with an electrostatic transducer
that is either externally or electret polarized.
Despite these efforts of the prior art, no transducer structures
using high polymer piezoelectric films have approached in
performance characteristics the full potential offered by the
intrinsic properties of such films.
SUMMARY OF THE INVENTION
The principal object of the present invention is to provide an
improved transducer of the type using a piezoelectric film as the
transduction element. Further objects of this invention are to
provide a piezoelectric thin film transducer that (1) is
elastically stable in operation and use, (2) employs no static
pressure difference on the film transducing element, (3) has an
electromechanical coupling coefficient approaching that of the film
itself, and (4) has minimal non-linearity of operation. Still
another object of the invention is to provide a transducer
structure which is more suitable for commercial manufacture and
use.
In a particular embodiment of the invention to be described
hereinbelow in detail, the transducer converts between electrical
and mechanical energy by means of the piezoelectric effect in a
supported sheet of uniaxially oriented, electrically polarized thin
high polymer film with surface electrodes thereon, and is
characterized in that the piezoelectric film is arranged with a
series of curved substantially cylindrical segments of alternating
sign of curvature, each of which is capable of functioning as a
separate and distinct transducer element. The surface electrodes on
the film, moreover, are divided, i.e., gapped, in selected
locations between adjacent segments to interrelate electrically the
separate transducer elements in a prescribed fashion, i.e., series,
parallel, or series-parallel. When the number of segments is even,
the second and other even-order harmonic distortion substantially
cancels, and in any case improved linearity of operation results.
At the same time, the provision of a number of supported
cylindrical segments or arches confers elastic stability without
the need for any static pressure means.
In further aspects of the invention, the piezoelectric film is
supported upon a frame which has a pair of spaced surfaces
containing curves of alternating curvature, across which the film
is conformingly secured under fixturing tension, and the frame has
ribs which support the film in locations between adjacent curved
segments. The fixturing tension may or may not contribute
significantly to the elastic stability of the film surface within
the segments. When used for electroacoustic purposes, the
transducer frame and film may be mounted within a casing having an
acoustic opening therein.
Other objects, aspects and advantages of the invention will be
disclosed in, or apparent from, the description hereinbelow of a
preferred embodiment as illustrated in the accompanying
drawings.
DESCRIPTION OF THE DRAWINGS
FIG. 1 is an exploded perspective view of a support and a sheet of
piezoelectric film arranged for use in an electromechanical
transducer in accordance with the present invention, but with other
transducer elements omitted for clarity;
FIG. 2 is an enlarged sectional view of the piezoelectric film
arrangement of FIG. 1, as taken on line 2--2 of FIG. 1;
FIG. 3 is a schematic sectional view of another piezoelectric film
according to the invention illustrating a parallel connection of
transducer elements;
FIG. 4 is a schematic section view of still another piezoelectric
film according to the invention, illustrating a particular
series-parallel connection of transducer elements;
FIG. 5 is a plan view of an electroacoustic transducer using a
piezoelectric film supported in accordance with the present
invention, with the transducer casing omitted, and
FIG. 6 is a section on line 6--6 of FIG. 5, showing the transducer
casing in dotted lines.
DESCRIPTION OF THE PREFERRED EMBODIMENT
Referring to the drawings, FIGS. 1 and 2 illustrate a piezoelectric
thin film 1 arranged pursuant to the present invention for use in
an electromechanical transducer, and a frame 2 which supports the
piezoelectric film 1. For clarity of illustration, FIGS. 1 and 2
omit other transducer elements.
The piezoelectric film 1, as is typical, is constructed with a
central polymer layer 3, e.g., of polyvinylidene fluoride, between
an upper surface electrode 4, and a lower surface electrode 5. For
clarity, the piezoelectric film 1 and its constituent elements are
shown with exaggerated thickness. The electrodes 4 and 5 may be of
vacuum metalized aluminum, and are very thin so that the low
mass/area ratio of the film is not significantly increased. The
film 1 is homogeneously polarized in the thickness direction, and
the uniaxial direction or axis of the film extends in the plane of
the film parallel to arrow 6. As is well known, when such a
piezoelectric film is stressed or strained parallel to its uniaxial
direction 6, one polarity of electrical signal is developed across
surface electrodes 4 and 5 for compressive strain, and the opposite
polarity is developed for tensive strain, and the piezoelectric
effect is greatest for this orientation of stress or strain in the
plane of the film. In certain instances a more complex laminate
structure than shown in FIGS. 1 and 2 may be desirable; for
example, to improve adhesion between the electrodes and the
piezoelectric film. Laminate structures of the latter type are
described in U.S. Pat. No. 3,912,830.
In accordance with the present invention, the piezoelectric film 1
is formed into a series of similar curved cylindrical segments,
such as S1 through S6, having alternating sign of curvature.
Preferably the segments are substantially congruent in
cross-section one to another. Thus, as seen from the top, the
piezoelectric film 1 has segments S1, S3 and S5 of convex curvature
alternating with segments S2, S4 and S6 of concave curvature. The
axes of the cylindrical segments extend perpendicular to the
uniaxial direction 6 of the film. In the embodiment illustrated in
FIG. 1 six segments are formed, each preferably comprising,
approximately, a portion of a right circular cylinder. Other curved
cylindrical shapes capable of suitably converting pressure
differences across the film 1 into stress along the uniaxial
direction 6 of the film 1 can also be employed. The number of
segments to be formed depends on the size of the transducer, and
the effective operating pressure it must withstand. In general, the
span l of each cylindrical segment is selected in relation to the
mechanical properties of film 1 so that each cylindrical segment is
elastically stable without mechanical bias over the range of
effective pressure differences to be encountered.
As shown in FIGS. 1 and 2, the frame 2 supports the piezoelectric
film 1 about its outer periphery with end walls 2E and side walls
2S. The end walls 2E have upper surfaces formed into alternating
curved cylindrical segments which conform to the desired
cross-sectional shape of the film 1. In addition, the frame 2 has a
plurality of spaced longitudinal ribs 7 which extend parallel to
the cylinder axes and support the film 1 at the junctures of
adjacent cylindrical segments S1, S2, etc. The film 1 is thus
supported in the regions where the film curvature reverses from
concave to convex, and vice versa. As a result of the manner in
which the piezoelectric film 1 is supported, each cylindrical
segment forms a distinct transducer.
Accordingly, when a pressure difference is applied across the whole
film, the convex segments S1, S3 and S5 will develop electrical
signals with polarities opposite to the electrical signals
developed in concave segments S2, S4 and S6. Pursuant to the
present invention, the upper and lower surface electrodes 4 and 5
contain spaces or gaps 4G and 5G which divide the surface
electrodes in selected locations at the junctures of adjacent
segments, i.e., where the film curvature reverses. The spaces
typically are formed in a completely electroded film, as by
chemical or electrochemical etching.
By means of the spaces 4G and 5G the distinct transducer elements
represented by segments S1 through S6 may be usefully interrelated.
In the arrangement illustrated in FIGS. 1 and 2, the spaces in
upper electrode 4 are located between segments S1 and S2, S3 and
S4, and S5 and S6, and thus alternate with and are offset from the
spaces in lower electrode 5, which are located between segments S2
and S3, and S4 and S5. The resulting portions of upper electrode 4
overlap with the portions of lower electrode 5, to place all of the
transducer elements electrically in series. Because of the reversal
of curvature between neighboring segments such as S1 and S2, the
transducer elements are placed in series in such a way that the
electrical signals developed across the piezoelectric transducer
elements are additive and produce a total electrical signal voltage
at output terminals 8 and 9 that has a magnitude of approximately
six times the signal developed across a single segment. The effects
of leakage capacitance reduce this multiplier somewhat from the
ideal value of six.
Suitable interrelation of the segments, such as the additive series
connection of film segments S1 through S6 illustrated in FIGS. 1
and 2, has an important advantage in that it improves transducer
performance and linearity by cancelling even order harmonic
distortion. At high acoustic pressures the maximum displacements of
the transducer elements can become significant relative to the
height of the cylindrical arches. For example, downward
displacements decrease the height of the convex arches but increase
the height of the concave arches. In any one transducer element,
the corresponding asymmetry of operation under positive and
negative displacements causes asymmetry in the mechanical coupling
between pressure difference on the film and stress in the uniaxial
direction of the film. In consequence the dominant non-linearity of
a single transducer element appears in the form of second order
harmonic distortion. However, the phase of the second (and all
even-order) harmonic distortion is opposite between convex and
concave transducer elements. Thus if all transducer elements are
similar with the exception of sign of curvature, and if the number
of concave elements equals the number of convex elements, the
even-order harmonic distortions associated with the individual
segments are placed in opposition, and the even-order harmonic
distortion substantially cancels in the total electrical signal
voltage at output terminals 8 and 9 of the multi-element
transducer. In this way the non-linearity of the transducer is
minimized, the remaining non-linearity appearing substantially as
third and higher odd-order harmonic distortion.
The transducer elements S1 through S6 need not be placed in series
however. As shown in FIG. 3, spaces 4G and 5G may be provided in
electrodes 4, 5 so that each transducer element, for example S1,
has a separate pair of electrodes. All electrodes of one electrical
polarity may be interconnected by electrically conductive means,
formed in part if desired by the pattern of etching of the surface
electrodes 4 and 5, extending near one edge of the film parallel to
the direction 6, and all electrodes of the other electrical
polarity may be similarly connected by means near the opposite edge
of the film, as indicated schematically by the connections shown in
FIG. 3 leading to output terminals 10A and 10B.
In addition to the series connection of elements shown in FIG. 1
and the parallel connection shown in FIG. 3, a number of
series-parallel connections can be utilized. An example is shown in
FIG. 4, wherein a series connection is made between adjacent
elements S1 and S2, S3 and S4, and S5 and S6, and the three sets of
series-connected elements are then connected in parallel.
Series-parallel connections are of some importance because the
number of transducer elements may be quite large, and the required
impedance level of the transducer may not be compatible with an
all-series or all-parallel connection of elements.
FIGS. 5 and 6 illustrate a complete electromechanical transducer
structure of a type suitable, for example, for use as a miniature
hearing aid microphone, In the transducer a piezoelectric film 11,
similar to film 1 shown in FIG. 1, is bonded by adhesive to a
support 12 which is molded from an electrically insulating plastic.
The support 12 has an integral frame which on its top surface has
flats 15 and 16, and non-flat surfaces 17 and 18 which, like the
upper surfaces of end walls 2E shown in FIG. 1, follow the contour
of film 11 as shown in FIG. 6. The support 12 also has integral
ribs 13 having top surfaces 19 which are straight along the ribs
but which follow the surfaces 17 and 18 transverse to the ribs. The
support 12 has perforations 14 which provide communication between
the underside of film 11 and air space 28.
Prior to assembly on the support 12, the film member 11 is longer
than shown in FIG. 5 so that fixturing tension may be applied to it
parallel to the ribs 13. The film member 11 is then brought down
onto the adhesive coated support 12, the tension serving in part to
conform the film member to the surfaces 17, 18, and 19 and portions
of 15 and 16, and to force the film member to take on in cross
section substantially the form of the surfaces 17 and 18 throughout
the length of the ribs 13 (as indicated in FIG. 6). The rib
surfaces 19 substantially face the electrode spaces 4G and 5G of
FIG. 1, so as to minimize capacitive shunting of the transducer by
those portions of the film member 11 rendered electromechanically
inactive by bonding to the surfaces 19. After completion of bonding
the film member 11 is trimmed to the length shown in FIG. 5, and
some tension remains in the film member parallel to the ribs
13.
A ceramic substrate 22, bearing on its bottom surface fired
conductive coatings forming leads to an encapsulated junction field
effect transistor (JFET) source follower 23, is adhesive bonded
into a recess in the support 12. Conductive coatings 20 and 21,
e.g., silver pigmented epoxy coatings, extend along channels in the
frame of support 12 to make contact between the outermost
electrodes of film member 11 and corresponding leads on the
substrate 22. The coating 21 connects to the gate lead of the
source follower, and accordingly the channel for 21 is deeper to
reduce leakage capacitance to a metallic casing 25 and cover 26
around the film 11 and support 12. The casing 25 and cover 26,
which define an acoustic cavity on each side of the film 11, are
shown dotted to indicate a particular electroacoustic transducer
incorporating this electromechanical embodiment of the invention.
The casing cup 25 is drawn with a slight land on which the support
12 rests for fixturing purposes. The space 24 in one of the
channels between the support and the casing wall is sealed except
for the provision of an acoustic vent or other acoustic impedance
element. For example, the space 24 may be sealed by a non-corrosive
silicone sealant, and the vent formed by the withdrawal of an
abhesive surfaced monofilament or wire fro the curved sealant. The
remainder of the support 12 is sealed and bonded to the wall of the
casing 25 by an adhesive such as epoxy, and the cover 26 is
similarly sealed and bonded to the support 12 and top edge of the
casing 25, the support 12 serving to locate transversely the cover
with respect to the casing. The cover 26 contains an acoustic inlet
27, which may contain or be overlaid by acoustic damping means, and
which functions as an acoustic connection through the wall of the
casing to the acoustic cavity within the casing above the film 11.
Wire leads extend from the substrate 22 to terminals in the casing
wall, indicated schematically by 29, 30, 31. Terminal 31 connects
to the drain and terminal 30 connects to the source of the JFET
contained in the source follower 23. DC electrical power may be
supplied by a cell or other means to the terminal pair 31 and 29,
in which case the electrical signal output from the source follower
is developed between terminals 30 and 29.
The electroacoustical transducer indicated in FIG. 6 is an
omnidirectional pressure operated microphone. Acoustic volume
displacement entering or leaving the inlet 27 causes mechanical
displacement of each of the cylindrical arches formed by the film
member 11 between any adjacent pair of the ribs 13, and these
displacements have approximately the same phase. Thus for example
downward displacements place in compression, in the direction
roughly corresponding to 6 in FIG. 1, the film portions that form
convex arches, and place in tension the film portions that form
concave arches. The resulting alternation of electrical signal
fields between adjacent transducer elements, generated by the
piezoelectric effect in the thickness direction of the
homogeneously polarized film, is precisely that needed for the
all-series electrode configuration of FIG. 1. The conductive
coatings 20 and 21, acting like the leads to the terminals 8 and 9
of FIG. 1, convey the total electrical signal voltage to the input
of the source follower 23, which in turn drives the output
terminals 30 and 29.
The elastic stability required in a practical transducer is
obtained in this invention by (1) the flexural stiffness of the
film as supported and fixed, or (2) by a combination of such
flexural stiffness and that tension in the film which remains after
fixturing and assembly to the support.
The critical pressure difference for the onset of elastic buckling
of a cylindrical arch is strongly dependent on its span because of
the effect of flexural stiffness. Indeed, in the absence of
flexural stiffness the critical pressure difference is zero if in
addition there are no membrane stresses that oppose buckling. As
indicated in FIGS. 1 and 6, the critical pressure difference for a
thin high polymer piezoelectric film is raised to a useful value by
dividing the extent of the film member 11 into a number of arches
of relatively small span, each arch being substantially supported
and fixed by an adjacent pair of ribs 13.
The use of flexural stiffness as the primary means of obtaining a
useful critical pressure difference incurs some reduction in the
effective electromechanical coupling coefficient of the film
transducer. Indeed, any means of obtaining sufficient elastic
stability will cause a reduction in effective electromechanical
coupling coefficient. However, the use of flexural stiffness is
highly effective and easy to control in raising the critical
pressure difference, and so the reduction of coupling coefficient
is minimized while the low mass/area ratio of the film is fully
preserved.
The electromechanical coupling coefficient of the film is also more
nearly realized by a natural consequence of the structures of this
invention. As illustrated in FIG. 5, the transducer elements are
very elongate, while the uniaxial direction 6 is transverse to the
long dimensions. At the ends of each element the film member 11 is
constrained by the support 12 at the surfaces 17 and 18 such that
the film cannot be strained appreciably in the uniaxial direction.
The effect of this constraint decays rapidly away from the ends of
the element, but the net effect is to create a "dead" portion of
the element that is electromechanically inactive but, if the
element is completely electroded over its length, has electrical
capacitance in shunt with the "active" portion of the element. This
equivalent to capacitive shunting reduces the effective
electromechanical coupling coefficient, but in this invention the
effect is minimal because, as a direct consequence of the highly
elongate shape of each element, the area of the "dead" portion is
small relative to the area of the element.
FIG. 6 illustrates an omnidirectional microphone embodiment of this
invention, but the same electromechanical assembly, and its
associated source follower if desired, is applicable to a variety
of directional microphone structures, the film member 11 being used
to sense pressure difference between the two acoustic spaces on
each side of it.
The electromechanical transducer of FIGS. 5 and 6 is also
applicable to headphone and loudspeaker tweeter applications,
provided that the excursion of the film element 11 is not required
to exceed roughly the film thickness and therefore that an
acceptable degree of linearity can be obtained. In addition, in
such applications the all-parallel connection of FIG. 3 may replace
the all-series connection of FIG. 1 in order to reduce the required
electrical signal driving voltage as much as possible. For example,
in a device in which the film element of FIG. 3 is substituted for
the film element 11 of FIGS. 5 and 6, the electrical input signal
is applied to terminals corresponding to 10A and 10B from a
transformer, electronic amplifier, or other means. This signal
causes the transducer elements to vibrate approximately in phase
acoustically, and the resultant acoustic volume displacement causes
acoustic pressure in a cavity or the radiation of acoustic energy
outward from the transducer, as the case may be. The discussions
above concerning elastic stability, effective electromechanical
coupling coefficient, and even-order harmonic distortion
cancellation are applicable to these sound generating embodiments
of the invention as well.
Although a specific embodiment of the invention has been disclosed
herein in detal, it is to be understood that this is for the
purpose of illustrating the invention, and should not be construed
as necessarily limiting the invention, since it is apparent that
many changes can be made to the disclosed structures by those
skilled in the art to suit particular applications.
* * * * *